Modern automotive engines rely on inductive ignition systems to fire the spark plugs. These comprise a coil having a secondary winding connected to a spark plug and a primary winding and a transistor switch serially connected across a voltage source. In operation, the switch is closed for a dwell period allowing current flow in the primary side to store energy in the coil, and when the switch is opened at the end of the dwell period the primary current is abruptly stopped and the secondary voltage becomes large enough to fire the spark plug, thereby discharging the stored energy into the plug gap. An undesirable side effect of the operation is that upon initial closing of the switch a high voltage, in the opposite sense, is developed in the secondary and typically is accompanied by oscillations or ringing of such magnitude that premature firing of the plug can occur, sometimes causing combustion in the corresponding cylinder. This phenomenon is called "ignition on make."
Ignition systems employing a distributor do not exhibit ignition on make because the additional gap provided by the rotor increases the required secondary voltage to a value higher than that due to the initial ringing. Similarly, distributorless ignition systems which have one coil shared by two cylinders also have an additional gap because there are two spark plugs in series and the premature firing does not occur. The ignition on make problem does appear, however, for ignition systems having one coil per cylinder; there the single plug gap can discharge at the beginning of the dwell period causing premature combustion.
One proposed solution to the ignition on make problem is to insert a high voltage blocking diode in the secondary circuit in series with the spark plug and coil to block the negative voltage produced at switch closing. Such diodes are expensive and potentially unreliable. Another solution is to slowly ramp on the transistor switch to reduce secondary ringing, but the dwell time delivered will not equal the dwell time commanded. The ramp introduces a delay which varies with temperature, battery voltage, and process variations.